ABSTRACT The importance of network security has grown tremendously and intrusion prevention/detection systems (IPS/IDS) have been widely developed to insure the security of network against suspicious threat. Computer network intrusion detection and prevention system consist of collecting traffic data, analyzing them based on detection rules and generate alerts or dropping them if necessary. However IPS has problems such as accuracy signature, the traffic volume, topology design, monitoring sensors. In this paper, we practically examine the traffic effect on performance of IPS. We first examine the detection of DOS attack on a web server by IPS and then we generate network traffic to see how the behavior of IPS has influenced on detection of DOS attack.